Processes for making paper pulp consist in reducing the raw materials to separate fibers containing a greater or lesser amount of cellulose depending on the qualities which the pulp produced is required to have.
The processes essentially consist of grinding operations, which are basically mechanical, which may be combined with more or less powerful delignification operations, which are basically chemical.
Depending on the relative importance of the two treatments, it is possible to distinguish five major types of pulp:
Mechanical pulp, obtained by grinding without any chemical treatment beforehand of the raw material; PA1 Thermo-mechanical pulp, obtained by grinding under pressure, which is made easier by steaming the raw material beforehand to soften the lignin; PA1 Mechano-chemical pulp, obtained by grinding in combination with in situ or ex situ preliminary treatment of the raw material with chemical reagents; PA1 Semi-chemical pulp, obtained by grinding raw material which is previously subjected to partial chemical "cooking" under pressure; PA1 Chemical pulp, where the chemical processing is much more powerful and produces both the delignification and the major part of the reduction to fibre.
As one passes from the mechanical pulp proper to the chemical pulp proper, the relative importance (and the difficulty) of the grinding in the manufacturing process decreases, and the mechanical properties of the pulp increase. At the same time, however, the ratio weight of pulp/weight of raw material decreases, and the process becomes more of a pollution problem, because of the presence of lignin and other vegetable extraction product solutions that require treatment.
Various parameters, especially the diminishing traditional forestry resources and the fight against pollution, have forced pulp producers to look for continuous improvement in the ratio of quality to output, especially by capitalizing on mechanical, thermo-mechanical and semi-chemical pulps.
This has produced disc grinders in particular which can grind wood chips by subjecting them to combined compression and shearing loads.
These grinders produce mechanical pulps which, for the same output, are more solid than the usual pulps produced by grindstones, because of the greater amount of long fibers which they contain.
Moreover, these grinders process raw material in the form of chips, which enables irregularly shaped pieces of wood, especially sawmill waste, sawdust and hardwood, to be used for producing mechanical pulp, while the grinders using grindstones employed until the development of the disc grinder require straight logs of a particular size, usually from conifers.
Disc grinders are also used with advantage in the production of thermo-mechanical, mechano-chemical, semi-chemical and chemical pulps, in particular for two-stage chemical processes with intermediate grinding.
In these processes, however, they only act as mechanical grinders, treatment with steam or chemical reagents having to be carried out in other machines associated with the grinders.
Although disc grinders are now widely used in the industry, they are not ideally suited to the function they are called on to carry out. In practice, the chips orient themselves between the disc in random directions, especially with respect to the shearing forces. This fault is accentuated as the discs wear, and degrades the regular nature of the grinding, giving a high yield of shives or silvers which require more than one passage through the machine.
Rapid wear of the discs and less than perfect control of temperature contribute equally to the heterogeneous nature of the pulp produced.
Furthermore, these machines consume large quantities of energy. To produce one ton of mechanical pulp with a disc grinder entails the consumption of 1700 to 1800 kWh, as compared with about 1200 kWh/Ton for pulp of the same quality produced by a grinder using grindstones, and only a small part of this energy is used for disintegrating the wood chips.
Other disadvantages result from the mechanical design of the machines. They must be very strong, able to withstand large axial loads, maintain good parallelism and be capable of expanding without deforming.
On the other hand, it has been proposed for some years that screw machines should be used for processing incompletely cooked pieces of wood such as knots and screening rejects. These machines usually comprise inter-penetrating screws driven in opposite directions.
But even in their most highly developed form, these machines can only grind down material in which the forces connecting the fibres together have been greatly reduced by chemical processing. They are not used for producing mechanical, thermo-mechanical or mechano-chemical pulps, but only for processing incompletely cooked pieces of wood and screening rejects in the conventional preparation of chemical and semi-chemical pulps.
Thus all the known processes call for lengthy chemical processing or high levels of mechanical energy. Also, the pulp is in all cases prepared in a discontinuous manner in a number of successive machines, which are generally very bulky.